At present, signals recorded in a hard disk drive is read by detecting, by a magnetic sensor, a leakage magnetic field from recording bits written in a magnetic disk.
However, as the recording density has been improved, the size of a recording bit has become very small, on the order of about 10 nm. For this reason, compatibility between the downsizing of magnetic sensors and the improvement in sensitivity thereof has become indispensable. In particular, as the gap length between magnetic shields in a reader has become very narrow, the thickness of magnetic sensor has been desired to be 10 nm or less.
However, spin valve elements commonly used at present have a four-layer structure including a magnetization free layer, a nonmagnetic intermediate layer, a synthetic, magnetization pinned layer, and an antiferromagnetic layer. Since the antiferromagnetic layer for pinning the magnetization of the magnetization pinned layer requires a thickness of about 10 nm to have satisfactory magnetization-pinning characteristics in the magnetization pinned layer, it is not easy to reduce the entire thickness of the four-layer structure to 20 nm or less.
Spin valve elements have been attracting attention, since they may serve as thin magnetic sensors capable of being disposed in a 10-nm gap between magnetic shields.
However, as will be described later, the spin valve elements are difficult to be used in hard disk heads that read data from hard disk devices (HDDs; magnetic recording and reproducing apparatuses) with fine bits, difficult to be downsized, and easy to be affected by external factors. Furthermore, the reported spin valve elements detect the recording magnetic field of media using variations in electrochemical potential, which is caused by the spin accumulation, as an electromotive force by means of a voltmeter having infinite internal resistance. Thus, it is difficult to perform the detection with preamplifiers of currently-available HDDs.